Used in medical diagnostics and research to assess vitamin D status in the body. It serves as a reference standard in laboratory tests that measure 25-hydroxyvitamin D levels in blood, helping diagnose deficiencies or monitor supplementation effectiveness. Commonly used in immunoassays and liquid chromatography-mass spectrometry (LC-MS/MS) methods for accurate quantification. Also utilized in calibration of diagnostic equipment and development of vitamin D assays in clinical laboratories.